This invention relates generally to the manufacture of semiconductor devices and, in particular, to photolithographic contact and proximity printing of semiconductor microstructures.
Typically, semiconductor microstructures containing a repetitive pattern of device geometries are printed onto prepared substrates such as silicon wafer substrates. Generally, the substrate is partly covered by a mask and the mask-substrate combination is transilluminated by light radiation from an optical lens system. In proximity printing, the mask is positioned a finite distance from the substrate. In contact printing, the mask is placed directly in contact with the substrate. Often in contact printing, because of irregularities in certain areas of the surface of the substrate, the mask does not make contact with all areas of the substrate, and a gap or separation is produced between the mask and substrate at the "out-of-contact" areas. Whether this separation is intended as in the case of proximity printing or unintended as in the case of contact printing, such a separation often produces unwanted line-width errors (i.e., errors in the size and boundaries or edges of the images transferred to the substrate) due to Fresnel or near-field diffraction of light and to source anisotropy (i.e., non-uniformity in quanta or intensity distribution of light over the surface of the light source).
For purposes of producing well-defined, good-resolution, accurately-sized, accurately-shaped microstructure images when performing contact or proximity printing, it is essential that light from the optical lens system be applied uniformly over the exposed (unmasked) portions of the substrate and that diffraction be compensated for without sacrificing image intensity. However, present methods, such as the exposure variation and multiple source synthesis technique described, for example, in the article entitled Young's Interference Fringes by M. V. Klein, appearing at pages 186-189 of the book Optics, published by Wiley and Sons, Inc., New York, 1970, and in British Pat. No. 1,353,739 issued May 22, 1974, to John Kenneth Houston, either limit the intensity of the light upon the substrate or limit the uniformity of light upon the substrate (i.e., sacrifice intensity for uniformity or vice-versa). Generally, as the light source becomes more anisotropic or as the separation of mask and substrate becomes greater, the more pronounced this sacrifice becomes. In many prior art systems, not only is intensity sacrificed in favor of uniformity as a result of the use of source beam divergence (or beam pick-up) angles that are too small and the inability of the system to adequately utilize a significant portion of the doughnut shaped or toroidal source irradiance distribution, but, often, the system is limited to fixed beam-divergence angular values.
What is needed, therefore, is an optical lens system that is capable of providing uniformity in the distribution of light upon a surface, such as an exposed (unmasked) surface of a semiconductor substrate, and of producing good quality images with minimal line-width errors, all without substantial sacrifice of image intensity. The system should not be limited to fixed beam-divergence angular values, should be capable of large source beam divergence (or beam pick-up) angles (e.g., about twenty degrees), should be capable of utilizing a significant portion of the toroidal source irradiance distribution, and should not be unnecessarily limited by source anisotropy or by the distance of the mask from the semiconductor surface.